The present invention relates generally to a method and apparatus for combining and amplifying radio frequency ("RF") signals. In particular, the present invention relates to an RF communication system in which plural independent signals may be combined, amplified, and radiated through shared resources with reduced distortion.
In many RF communication systems, it is desirable to share the resources of portions of the transmission systems among various communication users, according to their needs to utilize the RF communication channels. Frequently, it is desirable that a single amplifier system be used to amplify plural signals, the input signals not being connected with or related to each other in any way, and being fed to one or more transmitting aerials. The plural independent RF signals fed to a single amplifier system at the same time are simultaneously amplified for transmission and fed to an antenna distribution network. Such RF signals may include two or more information modulated carrier wave signals based upon carrier waves of differing frequencies (the "fundamental frequencies"). Such systems are disclosed in the Myer U.S. Pat. No. 4,580,105 issued Apr. 1, 1986; Engelbrecht U.S. Pat. No. 3,383,618 issued May 14, 1968; Olver U.S. Pat. No. 4,560,945 issued Dec. 24, 1985; and Gerard U.S. Pat. No. 4,423,386 issued Dec. 27, 1983.
It is well known that the amplification of RF signals usually results in the generation of harmonics of the input signal. The harmonics are spurious signals or frequencies which are whole-number multiples of the basic signal frequency, i.e., twice, three times, four times, et cetera. For a single basic frequency, these harmonics are reduced in power level to the point at which they can often be ignored. However, the amplification of plural combined signals of quite different basic or fundamental frequencies inevitably gives rise not only to the harmonics of the fundamental frequencies, but also to the formation of what are known as "intermodulation products", i.e., extra spurious output signals related to combinations of the basic input signals and their harmonics. For example, where a signal of frequency F1 and a signal of frequency F2 are simultaneously amplified, there will be generated the harmonics 2F1, 3F1, 4F1, 5F1 . . . and 2F2, 3F2, 4F2, 5F2 . . . and also intermodulation products of the type F1.+-.F2, 2F1.+-.F2, 3F1.+-.F2 . . . . Frequently, so many of these intermodulation product signals are produced that they cause unacceptable interference with other signals over a very wide portion of the radio spectrum being utilized. The interference problem grows tremendously as the number of independent input signals increases because the number of combinations increases very rapidly as the number of signals increase. The number of possible significant intermodulation combinations for just four fundamental frequencies can exceed 12,000 and for six basic frequencies can be over one-quarter of a million.
The numerous intermodulation products mixed along with the desired signals are transmitted by the antenna and can desensitize RF receivers which are located nearby.
Previously, to prevent or reduce the radiation of intermodulation products, narrowband filters were used with either or both of the transmitting equipment and the receiving equipment. Such filters can be quite useful in narrowband communication systems in which the frequencies of the transmitters and receivers are well known and unvarying and in which amplifiers can be dedicated to the amplification of single fundamental frequencies. However, modern wideband architecture allows multiple exciters to be fed to a common antenna with no tunable filters between the power amplifier and the antennas. Such an architecture is particularly useful for frequency-hopping RF systems in which narrowband filtering is difficult to perform because of the settling time for filters and the need for frequency-hopping systems to frequently change the carrier or fundamental frequency of the transmitted signal.
To reduce the amount of intermodulation products which were transmitted, some prior art systems, such as those disclosed in the Olver and Myer patents, used adaptive feed-forward cancellation in which a portion of the input signal, prior to being amplified, is compared to the signal after combination and amplification to identify and cancel the intermodulation distortion products. Such cancellation techniques generally improve the intermodulation distortion problems; however, such techniques generally do not eliminate all intermodulation distortion, consume considerable power, involve the addition of complex circuitry, and, because of the nonlinearities in the feed-forward cancellation apparatus, introduce new sources of error to the RF output signal.
Within the amplifying and combining circuits of prior art and the present invention, it is frequently necessary to use wideband transformers. Often, such transformers spray a considerable amount of RF interference ("RFI") signals into other elements of the amplifier and communication system because of the buildup and release of electromagnetic fields surrounding the transformers. The sprayed RFI signals may be coupled into other components of the amplifier system and thereby distort the desired output signal. Accordingly, it is often a concern of RF system designers to utilize transformers which reduce the amount of RFI generated and available for unwanted coupling.
Many transformers of the prior art comprise a magnetic ferrite surrounded by one or more turns of a transmission line. Where the transmission line and the ferrite are adjacent, the transformer is efficient; however, where the transmission line is not adjacent the ferrite core (e.g., around the ends), the transformer is often ineffective and standing waves may be generated.
Accordingly, it is an object of the present invention to provide a novel method and apparatus for combining and amplifying RF signals with fewer interfering intermodulation distortion products.
It is another object of the present invention to provide a novel transformer apparatus which reduces the inefficiencies of known prior art transformers and which reduces the amount of RF interference sprayed for potential interference with other components of the amplifier and combiner system.
It is a further object of the present invention to provide a novel method and apparatus for amplifying plural RF signals with a relatively higher power efficiency when compared to prior art wideband amplifiers.
It is yet another object of the present invention to provide a novel method and apparatus to reduce the amount of intermodulation product distortion which is generated and transmitted by inductive circuit elements within the amplifier system.
It is still another object of the present invention to provide a novel method and apparatus to reduce the voltage standing wave ratio at the output of an RF amplifier which is amplifying a combination of fundamental frequencies.
It is still a further object of the present invention to provide an RF signal amplification method and apparatus for use in applications where transmitting and receiving antennas must be in close proximity.
These and many other objects of the present invention will become apparent upon reading of the following detailed description of the invention when considered in conjunction with the drawings.